Ryan,
Sorry to hear about the keyboard. You might consider using a cheap USB hub when prototyping so you can fry the ports with impunity.
via: http://www.sparkfun.com/datasheets/Robotics/Ardumoto_v12.pdf
It looks like the ardumotor draws its power of Vin, and the two inverters draw from the Vcc line (which on Maple is 3.3V, I assume thats OK for those though).
On Maple Vin is connected to the input of the onboard regulators, which is determined by your jumper setting. The power selection jumper will source Vin with either the external barrel, the USB 5V, or the lipo input.
If you did not change the jumper, and were still powered off USB, then Vin would be sourced from USB - the ardumotor would draw from that line (500mA max) and likely trigger the overcurrent protection circuit on your computer's USB controller. If this is the case, the solution is to switch the jumper from USB to Ext.
Please also note, from the errata (http://leaflabs.com/docs/hardware/maple.html#errata) that the barrel jack is 12V max, not 18 as indicated on the silkscreen.
If you had the jumper set correctly, then its back to the debugging drawing board ;)
